Planning a trip through Vietnam myself. The best map I found is from Reise Know How. It's a German map, very detailed, plastified and untearable. I found it in a travelshop here in Holland but you can also order it online at http://reise-know-how.de/start-frames.php if you like to study it before you go, but I guess you can find some pretty good maps in Vietnam too.
Have a nice ride!
Rob